#### Answer

The amount saved is increasing, on average, by $\$50/month$.

#### Work Step by Step

To calculate the average rate of change between two points $(x_1,y_1)$ and $(x_2,y_2)$, we use the formula:
$\displaystyle average~rate~of~change=\frac{y_2-y_1}{x_2-x_1}$
We calculate the average rate of change with the points $(0,0)$ and $(4,200)$:
average rate of change$=\frac{200-0}{4-0}=\frac{200}{4}=50$ dollars/month
Therefore, the amount saved is increasing, on average, by $\$50/month$.